Fiber laser devices are used in the fields of processing machinery, medical equipment, measurement instruments, and the like, and light that is amplified by amplification optical fibers is output from the fiber laser devices. In such a fiber laser device, there are cases where light that is output from the core of the amplification optical fiber is input to the core of a delivery fiber, and propagates through the delivery fiber. After reaching a desired location, the light is output.
However, output light that is output from the delivery fiber might be reflected by an object to be processed or the like, and be input to the clad of the delivery fiber. Further, part of light that is input to the delivery fiber might leak into the clad due to axis misalignment or angle mismatch between the cores, a mode field difference, a core bend, or the like at the welded point. In that case, the light that is input to and propagates through the clad might be absorbed by the coating layer of the delivery fiber, and the coating layer might burn out.
Patent Document 1 listed below discloses a system in which light that is input to such a clad is released from an optical fiber. This system includes a member having a refractive index that is a negative temperature coefficient and conforms to the refractive index of the clad, and the outer peripheral surface of the clad of the optical fiber is coated with the member. Where light is input to and propagates through the clad, the light propagates toward the member, and is then removed from the clad.